In the letter, Sen. Grassley warns that any attempt to retaliate against the cooperating official, Assistant Special Agent in charge of ATF’s Phoenix Division George Gillett, is unlawful. Prior to Sen. Grassley’s letter, Gillett had already told his supervisors, through his attorney, that he was cooperating.

Sen. Grassley also says the apparent efforts of ATF executives to stop employees from speaking with members of Congress and their staff is of “grave concern.”

“Without such direct, unfiltered communications, Congress would still be unaware of, and unable to inquire about, the serious allegations involving the death of Border Patrol Agent Brian Terry and the sales of weapons to known and suspected gun Traffickers,” reads the Sen. Grassley letter.

More at the link.

The pdf linked above has that ATF email obtained via a FOIA request. In it, you will see that as recently as February, the ATF was denying any connection between the death of Border Patrol agen Brian Terry and their “Fast and Furious” guns.